In the manufacturing of integrated circuits, photoetching is used for reproducing the image of a pattern drawn on a mask. This image is formed on a thin layer of photoresist itself deposited onto a layer of a product to be specifically etched, for example a silicon dioxide layer. After having been exposed, irradiated areas of the resist are dissolved by a suitable developing material, in case of a positive photoresist. Conversely, the non-irradiated areas are dissolved in case of a negative resist. In the present application, only the case of a positive photoresist material shall be explained, but it is clear that it applies conversely to the case of a negative photoresist material.
FIG. 1 illustrates a basic projection masking method. The image of a mask 1 is formed through an optical system 2 on an area 3 of a wafer 4. The mask may correspond to the whole wafer or comprise the drawing of an elementary integrated circuit which is repeated a plurality of times on the wafer by moving same at successive positions. The mask 1 is usually at a scale ranging between 1 and 10 with respect to the image to be obtained on the wafer.
An example of an operation made on a wafer is illustrated in FIG. 2. The image of a mask comprising a dark layer with a slot-shaped aperture is projected onto a wafer comprising for example a silicon substrate 10 coated with a silicon dioxide layer 11 (SiO.sub.2), itself coated with a photoresist material layer 12. The photoresist layer is illuminated at the image of the slot and, further exposed to etching by a selected material, so that the irradiated area is eliminated for obtaining in the photoresist an aperture 13. The slope of the edges of this aperture is, in a large proportion, determined by the characteristics of the exposure system and mainly by the aperture number of the projection system.
After etching the photoresist layer, it is possible to form an aperture 14 into the underlying silicon dioxide layer, for example by plasma etching. In case of an anisotropic plasma etching, the slope of the edges of the aperture 14 in the layer 11 will depend upon the slope of the edges of the aperture 13 in the photoresist layer 12. It is known that if B2 is the slope of the edges of the aperture 14 and B1 the slope of the edges of the aperture 13, tan B2=(V2/V1) tan B1, V2 and V1 being the respective etching rates in the plasma of the photoresist material and the silicon dioxide (or any other material forming the layer 11). Therefore, by controlling the slope of the apertures formed in the photoresist material, it will be possible to control the slope of the apertures formed in the underlying layer. The latter slope presents an important practical interest. Indeed, in the above example, if the aperture 14 corresponds to a metallization contact and a connection by means of, for example, an aluminum metallization is to be made, if the edges of aperture 14 are too stiff, the aluminum layer is liable to crack or not to cover the upper corners of aperture 14 in the silicon dioxide layer 11. This risk is reduced if the aperture is flared out.
The control of the slopes presents another important practical consideration. The automatic alignment of exposition machines very often uses the contrast of a mark etched into the wafer at a former masking level. Such systems are generally sensitive to the thickness of the underlying layers. If the edges of those marks are flaring too much, this sensitivity is reduced due to the diffraction increase.
In case the apertures to be formed are greater than several micrometers, a plurality of methods exist for softening the slope of the apertures formed in the photoresist material. For example, curing after developing is possible, which causes the photoresist material to flow and rounds up the upper angles of the apertures. On the contrary, when the technology used causes the minimum dimensions of some patterns to be in the range of one micrometer, the thickness of the photoresist layer is in the same range as the width that has to be maintained. In this case, the flowing technics are no longer suitable because some effects associated with surface stresses are caused and, for example, a rib of one micrometer width of photoresist material, instead flowing over the underlying layer will possibly concentrate to attain an ovoid shape.
In the absence of techniques such as curing, the slope of the aperture in the pattern is determined by the characteristics of the exposure optical system and in fact mainly by the aperture number of this optical system. In case of patterns having sizes equal to or lower than one micrometer, one chooses systems with a large aperture number, for example in the range of 0.35 and its results therefrom that the slopes of the edges of the apertures formed in the photoresist material are naturally in the range of 80.degree. or more.